The present invention is related to a sound dampening material for use indoor in buildings such as apartments, hospitals, shopping centers where people reside or move with the aim of dampening sound.
Numerous devices and materials for damping sound and noise in buildings are known from the prior art. One example can be found in U.S. Pat. No. 5,740,649 which describes a false ceiling for buildings designed to absorb acoustic waves. The ceiling is made up of hard plates of metal or plastic perforated with holes with a diameter of 0.2-3 mm. The plates are suspended in the ceiling. Another example of a sound absorbing material can be found in U.S. Pat. No. 3,094,188. This patent describes slabs to be mounted to for example a wall in a building. The slabs comprise a porous material perforated with recesses in the form of holes or slits with a given shape and depth to provide the desired acoustic impedance where the slab is to be mounted. FR 1 233 707 is a related publication. Yet another example of a sound dampening material can be found in U.S. Pat. No. 3,820,628. This patent describes through slits provided in the surface of a part of an air propulsor. Finally, EP 1 861 554 A1 describes a sound absorbent of a hard material, such as metal, glass or plastic in the form of panels provided with through microslits. U.S. Pat. No. 6,194,052 describes a sound absorbing sheet material of metal provided with numerous through microslits cut into the material. The microslits are produced by stamping or punching, which leaves an uneven surface which is susceptible to dust collection.
The sound dampening effect achieved by the apertures in the material is in general caused as follows: air in the apertures is put into vibration by the sound, whereupon the energy in the sound waves is converted into heat due to the friction of the viscous air flow in the apertures. To obtain this vibration of air in the slits, the sound absorbing material with its apertures is arranged at a certain distance from the object it is attached to, such as a ceiling. Then the air between the sound absorbing material and the object will fluctuate due to acoustic vibration. Accordingly, the sound dampening effect is obtained by a combination of viscous dissipation of the sound energy and Helmholtz absorption. The technology related to the sound dampening effect of constructions with apertures as mentioned above is not described in further detail here.
US 2001/0050197 A1 discloses a sound absorbing microperforated polymeric film. The material is embossed by a tool having posts. The embossed holes may for example be circular, square or hexagonal. There is no mention of any slits. However, the mechanical embossing process leaves deflections at the edge of the opening, providing an uneven surface that is more subject to dust collection than a level surface.
The article “Properties and Applications of Microperforated Panels” by Herrin et. al. is discussing micro perforated panels as acoustic absorbers. On page 6 it is stated that “Slit-shaped perforations have a slightly smaller acoustic resistance but function similar to circular holes for all practical purposes”. Accordingly, the art suggests the use of holes instead of slits.